In a spark plug, a metal shell mounted to an engine holds an insulator made from ceramic such as alumina (Japanese Patent Application Laid-Open (kokai) No. 2014-107084). If moisture (condensate or the like) or fuel (hereinafter, referred to as “moisture or the like”) is put on the surface of the insulator of the spark plug mounted to the engine and the moisture or the like reacts with a glass phase at a grain boundary of the ceramic, there is a risk that the glass phase is degraded and the strength of the insulator is reduced.
In the above-described conventional technique, there is a need to suppress reduction in the strength of the insulator due to reaction between the moisture or the like and the glass phase.
The present invention has been conceived to address the above-described need, and an object of the present invention is to provide a spark plug enabling suppression of reduction in the strength of an insulator.